Nanowires are wires having diameters ranging up to several hundred nanometers. Nanowires are extremely sensitive to their surroundings and so are suited to a plethora of sensing applications. The fibre geometry provides extremely long optical path lengths and allows the use of small sample volumes.
Nanowires work by detecting changes in light, whereby the nanowire functions as a “rail” for the light since most of the light is situated in the surrounding cladding comprising holes filled with air or fluid, and thus the light can interact with any materials (gases, liquids, particles, etc) in the holes. Changes in the properties of the mode field of the light due to interaction with the material will be transmitted along the nanowire and can be detected at the output end of the nanowire device.
A nanowire has enhanced sensitivity compared with wires of larger diameter, since the power fraction of light in the environment is much larger for nanowires than for wires of larger diameter. The larger power fraction of light in the fluid surrounding the nanowires results in larger overlap of light and fluid, resulting in the enhanced sensitivity of nanowires.
Nanowires are however, difficult to fabricate. The fabrication of nanowires in the laboratory is a process which requires considerable skill, particularly since it is desirable to produce a wire in which the diameter does not fluctuate substantially over the length of the wire and the sidewall roughness is kept to a minimum. Diameter fluctuations, sidewall roughness and contamination can make the wires unsuitable for low loss optical wave guiding.
In order to have broad ranging commercial application, it is desirable that nanowires are of a sufficient length for practical use. For, example, the wires may be required as sensors, embedded into a structure having dimensions of the order of meters. However, known techniques for the fabrication of nanowires result in very short portions of wire, which are of scientific interest but have limited commercial viability.
One known technique for the formation of nanowires includes a two step drawing process. The first step involves the use of a flame to draw a silica fibre to micrometer-diameter wire. Secondly, one end of the drawn wire is placed horizontally onto a tapered tip (usually a sapphire fibre taper) and the tip is rotated about its axis to wind the silica nanowires. Using this technique, nanowires with diameters smaller than 50 nm can be obtained, however, the lengths of wires produced are only of the order of tens of millimetres. A problem anticipated for longer nanowires is that while they are extremely sensitive to their surroundings, they are fragile and prone to contamination during handling. One way of supporting a nanowire is to encase the wire in a porous substance such as an aerogel. This allows the wire to be handled, however, the disadvantage is that the arrangement is no longer flexible, which limits the commercial application as well as the ability to act as a sensor.
Nevertheless, a nanowire which is fully enclosed within a supporting structure loses the ability to perform point detection since the core may not be close enough to the surface walls.
It is an object of the present invention to address at least one of the problems discussed above.